Non-cryogenic current comparators are direct current comparator measurement devices which are not based on superconductive shielding. These devices are limited to a ratio uncertainty of about one part in ten million due to magnetic core noise. Moreover, windings of about one thousand turns are required to achieve this uncertainty.
I. K. Harvey is credited with development of the original cryogenic current comparator (CCC) device for operation at liquid helium temperature (see I. K. Harvey, "A Precise Cryogenic DC Ratio Transformer", Rev. Sci, Instrum., vol. 43, pp. 1626-1629 (1972)). This early comparator comprised a tube which provided Meissner-effect magnetic shielding between internal ratio windings and an external magnetic detector. Opposing currents in the ratio windings set up nearly canceling shielding currents on the surface of the tube, and a ratio balance at any integer ratio of currents could theoretically be detected and maintained using appropriate windings.
Present day CCC devices comprise liquid helium temperature overlapped tube-CCC devices which operate at 4.2.degree. K. These devices have been used since about 1975 to measure resistance ratios in some laboratories which maintain and disseminate reference standards for electrical measurements. These CCC devices typically include several sets of windings shielded by continuous overlapping layers of a metal foil or shield which is superconducting at 4.2.degree. K. The shielding can be nearly complete in overlapping tubes; however, the devices are complex and delicate, and as a consequence all such devices which are in use today are handmade.